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PEER-REVIEWED ARTICLE bioresources.com Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4810 Effect of Desulfonation of Lignosulfonate on the Properties of Poly(Lactic Acid)/Lignin Composites Hanzhou Ye, a,b Yang Zhang, a,b, * and Zhiming Yu a,b, * To utilize the lignin generated by the paper industry and reduce the cost of poly(lactic acid) (PLA), PLA/lignin composites were prepared from PLA and different ratios of lignosulfonate (LS) or desulfonated lignosulfonate (DLS) particles using a casting method. The physicochemical properties of the lignins were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel permeation-high performance liquid chromatography (GPC), and elementary analysis. The results indicate that the sulfur content of the original LS was successfully reduced to half by desulfonation to produce DLS, which was found to have a higher thermal stability and a lower average molecular weight than LS. Additionally, the thermal stability, crystallization, compatibility, mechanical, hydrophobicity, and optical properties of the PLA/lignin composites were also meticulously evaluated. Comparison of the PLA/DLS and PLA/LS composites revealed that the incorporation of DLS into PLA improved compatibility, thermal stability (T5% and Tmax), and hydrophobicity, while the mechanical properties remained almost unchanged. In addition, both PLA/DLS and PLA/LS exhibited UV light absorption capacity. Finally, the low-rate addition of both LS (10%) and DLS (5%) accelerated the crystallization of PLA, but crystallization was delayed with higher lignin content. Keywords: Lignin; Lignosulfonate; Poly(lactic acid); Casting; Desulfonation Contact information: a: Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing 100083, China; b: Ministry of Education Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Beijing 100083, China; * Corresponding authors: [email protected]; [email protected] INTRODUCTION Lignin is the second most abundant non-food biopolymer on the planet. The chemical structure of lignin is sometimes difficult to define, given that its structure and properties largely depend on the isolation process and the source type used for its extraction (Agrawal et al. 2014; Gordobil et al. 2014; Thakur and Thakur 2015). Commercially, lignin is mostly obtained as a by-product of the pulp and paper industry. However, most lignin is consumed as a fuel or energy source, and at present, only approximately 2% of the annually produced lignin is being commercialized (Saito et al. 2014). Recent industrial applications of lignin are primarily lignosulfonate (LS)-based materials (Liang et al. 2013). Most of the LS is precipitated from spent process liquid and black liquid from sulfite plants by the treatment with lime during the pulping process (Agrawal et al. 2014).
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PEER-REVIEWED ARTICLE bioresources.com

Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4810

Effect of Desulfonation of Lignosulfonate on the Properties of Poly(Lactic Acid)/Lignin Composites

Hanzhou Ye,a,b Yang Zhang,a,b,* and Zhiming Yu a,b,*

To utilize the lignin generated by the paper industry and reduce the cost of poly(lactic acid) (PLA), PLA/lignin composites were prepared from PLA and different ratios of lignosulfonate (LS) or desulfonated lignosulfonate (DLS) particles using a casting method. The physicochemical properties of the lignins were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel permeation-high performance liquid chromatography (GPC), and elementary analysis. The results indicate that the sulfur content of the original LS was successfully reduced to half by desulfonation to produce DLS, which was found to have a higher thermal stability and a lower average molecular weight than LS. Additionally, the thermal stability, crystallization, compatibility, mechanical, hydrophobicity, and optical properties of the PLA/lignin composites were also meticulously evaluated. Comparison of the PLA/DLS and PLA/LS composites revealed that the incorporation of DLS into PLA improved compatibility, thermal stability (T5% and Tmax), and hydrophobicity, while the mechanical properties remained almost unchanged. In addition, both PLA/DLS and PLA/LS exhibited UV light absorption capacity. Finally, the low-rate addition of both LS (10%) and DLS (5%) accelerated the crystallization of PLA, but crystallization was delayed with higher lignin content.

Keywords: Lignin; Lignosulfonate; Poly(lactic acid); Casting; Desulfonation

Contact information: a: Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry

University, Beijing 100083, China; b: Ministry of Education Key Laboratory of Wooden Material Science

and Application, Beijing Forestry University, Beijing 100083, China;

* Corresponding authors: [email protected]; [email protected]

INTRODUCTION

Lignin is the second most abundant non-food biopolymer on the planet. The

chemical structure of lignin is sometimes difficult to define, given that its structure and

properties largely depend on the isolation process and the source type used for its

extraction (Agrawal et al. 2014; Gordobil et al. 2014; Thakur and Thakur 2015).

Commercially, lignin is mostly obtained as a by-product of the pulp and paper

industry. However, most lignin is consumed as a fuel or energy source, and at present,

only approximately 2% of the annually produced lignin is being commercialized (Saito et

al. 2014). Recent industrial applications of lignin are primarily lignosulfonate (LS)-based

materials (Liang et al. 2013). Most of the LS is precipitated from spent process liquid and

black liquid from sulfite plants by the treatment with lime during the pulping process

(Agrawal et al. 2014).

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4811

Currently, the global production of LS is 1.1 million tons per year, and the market

for LS remains restricted and focused on soil stabilizers (Indraratna et al. 2012), concrete

plasticizers (Surendra et al. 2015), cement dispersants (Lummer and Plank 2012), oil well

drilling agents (Chen et al. 2014a), and adhesive agents (Brizius and Montgomery 2014).

In addition, it has also been mixed with certain conventional petroleum-based polymers,

such as polyurethane (PU) (Hatakeyama et al. 2013; Ferry et al. 2015), polypropylene

(PP) (Yu et al. 2012; Bozsódi et al. 2016), polyvinyl alcohol (PVA) (Li 2011; Ye et al.

2016), and polyethylene (PE) (Hu and Guo 2015). LS has also been used as an

antioxidant to increase the UV resistance of materials (Afrin et al. 2012; Yang et al.

2015) and as a flame retardant to increase polymer stability (Pan et al. 2016), which has

enhanced its value and expanded its use.

Admittedly, environmental concerns and a depletion of petroleum resources have

driven efforts to prepare bioplastics from renewable materials. Several studies have been

carried out on biopolymer/LS composites, with the aim of enhancing the mechanical

properties and crystallization (Lin et al. 2011; Duval et al. 2013) of the biopolymer

matrix. Poly(lactic acid) (PLA), a biodegradable and aliphatic polyester material, is

derived from plants, such as corn, sugar beets, and wheat. As it has been considered one

of the most promising environmentally-friendly, sustainable development polymers

(Gordobil et al. 2014; Alay et al. 2016), PLA is being extensively studied with the aim of

replacing commodity polymers. Although PLA is recyclable and compostable with good

stiffness and strength, it has its own obvious drawbacks, such as inferior moisture

sensitivity, early physical aging, poor impact resistance, and a relatively high price,

which limit its extensive use (Imre et al. 2013; Liu et al. 2016).

LS, used as a natural antioxidant and low-cytotoxicity additive, shows great

potential in food packaging (Núñez-Flores et al. 2012; Domenek et al. 2013) and in the

newsprint paper industry (Komissarenkov and Lukanina 2012; Pang et al. 2016).

Additionally, the inexpensive, renewable lignin is a good candidate to be a filler for PLA

composites and also to increase the antioxidant activity of polymers (Zafar et al. 2016).

However, with the incorporation of a hydrophilic LS filler, the water absorption of the

obtained composites inevitably increases (Pye 2008; Revin et al. 2016), leading to

accelerated biodegradation of the polymer/LS due to the increase of hydrophilicity (Lin et

al. 2011), which is disadvantageous for PLA/lignin composites. As a strong hydrophilic

group, sulfonic acid group plays a key role in the hydrophilicity of LS. Reducing the

content of sulfonic acid group in LS can lower the hydrophilicity of the PLA/LS, and thus

diminish the biodegradation of composites. The application of desulfonation of LS has

been reported in nitrogenous fertilizers (Fischer and Schiene 2002) and oil recovery

(DeBons et al. 1992). However, to the best of our knowledge, the desulfonation of LS in

polymer/LS composites has not been reported yet.

In this study, the number of hydrophilic groups of LS was lowered to produce

desulfonated LS (DLS), leading to the enhancement of the hydrophobic qualities of LS.

DLS was obtained by the desulfonation of LS, which greatly reduces the sulfonic acid

group and thus decreases the water sensibility of the resultant polymer. Byproducts that

are released, when lignin is heated, can impair polymer thermal stability (Sallem-Idrissi

et al. 2016). Thus, the effect of the process of desulfonation of lignosulfonate on the

properties of the resultant composites prepared by a casting method were investigated.

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4812

EXPERIMENTAL

Materials PLA 4032D (NatureWorks LLC, Minnetonka, MN, USA), contained less than 2%

D-LA with a molecular weight of between 220,000 and 240,000 g mol-1. The glass

transition temperature (Tg) and melting point (Tm) of this PLA are 58 °C and 160 °C,

respectively. The LS used in this study was supplied by Fuchen Chemical Reagents

Factory (Tianjin, China). All other reagents were of analytical grade and used as

received.

Desulfonation of LS

A predetermined amount of alkali was added to the LS solution to produce DLS

by hydrolysis. Fifty grams of LS and 4 g of NaOH were dissolved in 500 mL of distilled

water in a three-necked flask incubated in an oil bath at 180 °C for 3 h. Then, after the

solution was cooled to 90 °C, 200 mL of 15% H2SO4 were added to the flask and stirred

with a magnetic stirrer at ambient temperature for 1.5 h. Subsequently, the product was

collected by suction filtration, washed with distilled water, and dried in a vacuum drying

chamber at 40 °C until it reached a constant weight.

Composite Preparation Lignin particles (LS or DLS) were introduced into a PLA matrix to generate

PLA/lignin composites by a casting method. Briefly, totally dried PLA (2 g) was

dissolved in 20 mL of dichloromethane (DCM) with magnetic stirring at room

temperature. Different weight contents of LS or DLS (5%, 10%, or 20%) were added to

the PLA solutions obtained above and mixed evenly at room temperature by

ultrasonication at 50 Hz for 10 min. The solutions were cast onto a Teflon® mold (200

mm × 200 mm) and dried at room temperature for 24 h. Then, the obtained composites

were placed into a vacuum drying chamber at 40 °C for 48 h to totally remove the

residual solvent. The resulting dried composites were peeled from the mold, and their

thickness were approximately 30 μm. The designated codes of LS, DLS, and the

PLA/lignin composites are listed in Table 1.

Table 1. Nomenclature for the Obtained Lignins and PLA/Lignin Composites

Sample Code PLA loading (%) LS loading (%) DLS loading (%)

LS - 100 -

DLS - - 100

PLA 100 - -

PLA/LS5 95 5 -

PLA/LS10 90 10 -

PLA/LS20 80 20 -

PLA/DLS5 95 - 5

PLA/DLS10 90 - 10

PLA/DLS20 80 - 20

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4813

Characterization Fourier transform infrared (FTIR) spectroscopy analysis

FTIR spectra of the samples were recorded in the spectral range from 600 to 4000

cm-1 using a Vertex 70 FTIR spectrophotometer (Bruker, Ettlingen, Germany).

Gel permeation-high performance liquid chromatography (GPC) analysis

The average molecular weight (Mw) and polydispersity (Mw/Mn) of the lignins

were studied by gel permeation-high performance liquid chromatography (GPC) using a

Waters 1525 system (Waters Corp., Milford, MA, USA). Tetrahydrofuran (THF) eluent

was used as the GPC mobile phase, at a flow rate of 1 mL min-1 and 35 °C. A Waters

1424 pump index detector and a column PL gel 5 μm mixed-C (Agilent, USA) were

employed. Calibration was performed using polystyrene standards.

Elemental analysis

An elemental analyzer (KZDL-8A sulfur auto-analyzer, Tianguan Instrument Co.

Ltd., Henan, China) was applied for quantitative analysis of sulfur content in the LS and

DLS, using the colorimetric titration method, according to ASTM D3246-15 (2015).

Thermal properties test

The thermal degradation patterns of all the specimens were determined with a

Q50 thermogravimetric analyzer (TGA) (TA Instruments, New Castle, DE, USA). The

samples were vacuum-dried at 40 °C for 24 h before the test. Approximately 5 to 10 mg

of the sample was placed on a platinum cup, and the results were recorded in the range

from room temperature up to 600 °C, at a heating rate of 10 °C min-1 under a nitrogen

atmosphere. The peak temperature at the maximum degradation rate was calculated.

In addition, the glass transition temperature (Tg) of the samples was recorded by

differential scanning calorimetry (DSC). Several 3-mg samples were placed in sealed

aluminum pans; an empty pan was used as a reference. The samples were first heated to

100 °C and held at this temperature for 5 min, then cooled to 0 °C at a rate of 20 °C min-1

for 5 min. Subsequently, the samples were heated at 20 °C min-1 to a final temperature of

200 °C (second cycle). The Tg was determined according to the second heating run to

avoid the influence of the thermal history.

X-ray diffraction (XRD) analysis

The X-ray diffraction patterns of neat PLA and PLA/lignin composites were

determined with an X' Pert PRO MPD X-ray diffractometer (Philips, Amsterdam, the

Netherlands), using Cu Kα radiation (λ = 1.5406 Å). Scattered radiation was detected in

the range of 2θ from 5° to 55°, at a scan rate of 5° min-1.

Scanning electron microscopy (SEM) analysis

The cross-section (fractured in liquid nitrogen) and the surface morphology of the

specimens were examined by SEM. Samples were coated with gold by an auto-fine

coater (JFC-1600, JEOL Ltd., Tokyo, Japan) and characterized by field-emission

scanning electron microscopy and scanning transmission electron microscopy

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4814

(FESEM/STEM) on a JEOL JSM-6700F microscope (JEOL USA Inc., Peabody, MA,

USA) operating at 5 kV.

Contact angle

Water contact angle measurements were carried out using a contact angle

analyzer (OCA 20 DataPhysics Instruments GmbH, Filderstadt, Germany) with a 3 μL

dosing volume and a 1 μL s-1 dosing rate. All the specimens were pasted on a slide glass

before measurement. Five drops per sample were placed on the surface and measured.

UV−visible spectroscopy (UV−vis)

The transparency of the samples was determined by UV-Vis spectra recorded in

the absorbance region from 190 to 400 nm using a TU-1901 spectrophotometer (Beijing

Purkinje General Instrument Co., Ltd., Beijing, China), using neat PLA as a reference.

Tensile tests

Samples with the dimension of 80 mm × 10 mm (length and width) were prepared

and the tensile strength and Young’s modulus were recorded at 24 °C and 25% relative

humidity using an INSTRON 3365 universal testing machine (INSTRON, Norwood,

MA, USA), with a loading cell of 100 N and a speed of 2 mm min-1. Five replicates per

sample were used, and average values are reported.

RESULTS AND DISCUSSION Properties of LS and DLS FTIR spectra

A possible schematic of the desulfonation reaction of LS is depicted in Fig. 1. The

peak assignments for the infrared absorption bands for LS, DLS, and kraft lignin (KL),

are based on the characteristic bands of lignin reported in previous literature (Keshk and

Sameshima 2006; Suganuma et al. 2008; Lisperguer et al. 2009; Yang et al. 2014; Kim et

al. 2015; Shankar et al. 2015; Strassberger et al. 2015) and are shown in Fig. 2. The

spectrum of DLS is similar to that of KL. Aromatic ring bands and the C-H deformation

between 1515 and 1460 cm-1 are very similar for DLS and KL, but the peaks of LS are

ambiguous. The strength and broadening of these peaks were probably associated with

the overlap of neighboring peaks at 1591 and 1419 cm-1. However, the overlapping of

FTIR absorption bands often makes it difficult to perform direct peak assignments of

chemical constituents (Zhou et al. 2011).

Compared with LS, the basic structure of lignin remained in the DLS after

desulfonation. Bands centered around 2937 and 2850 cm-1 can be attributed to C-H

stretching vibrations of CH2, CH3, and tertiary CH groups in the DLS and KL. Another

peak at 1724 cm−1 in DLS and KL can be attributed to ester carbonyl vibrations from the

feruloyl, acetyl, and p-coumaryl groups in lignin.

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4815

Fig. 1. A possible schematic of the desulfonation reaction of LS

Fig. 2. FTIR spectra of LS and DLS

Molecular weight distribution and sulfur content

The weight averages of lignin differ greatly as a result of the feedstock and the

extraction process used (Gordobil et al. 2014, 2015). According to the literature, the

weight average (Mw) of lignin was detected from 649 to 10680 g·mol-1 and the number

average (Mn) was from 376 to 4900 g·mol-1 (Scholze et al. 2001). The values of the Mn,

Mw, polydispersity (Mw/Mn), and total sulfur content of both lignins are listed in Table 2.

In this case, as the value of the total sulfur content declined after desulfonation, the

molecular weight of the DLS was found to decrease due to the rupture of the bonds in LS.

Similarly, for both LS and DLS, a narrow molecular weight distribution was indicative of

a relatively low value of polydispersity (El Mansouri and Salvadó 2006; El Hage et al.

2009). Moreover, in our case, the total sulfur content of DLS was lower than that of LS,

indicating that the desulfonation process occurred.

Table 2. Number Average (Mn), Weight Average (Mw), Polydispersity (Mw/Mn), and Total Sulfur Content of LS and DLS

Sample Code Mn (g·mol-1) Mw (g·mol-1) Mw/Mn Total Sulfur Content (%)

LS 2317 2329 1.00 5.78 ± 0.58

DLS 604 635 1.05 2.32 ± 0.11

Thermal stability

SO3

ORH3CO

_

A

OH

D esulfonation

OH

ORH3CO

A

OH

LS D LS

R = H or LS

A = O , H or LS

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4816

The thermogravimetric analysis (TGA) was conducted from room temperature up

to 600 °C under a nitrogen atmosphere, and the derivative thermogravimetric (DTG)

curves of LS and DLS were obtained (Fig. 3). The DTG curves indicated that the

degradation of LS was more complex than that of DLS, with some peaks and plateaus

throughout the whole range of temperatures. The weight loss of the DLS was associated

with two main stages (Li and McDonald 2014). The degradation of LS occurred in three

steps, which is consistent with other research (Li et al. 2014).

The first stage of the degradation of DLS and LS began at approximately 150 °C,

while the second stage of the degradation of DLS was centered at approximately 250 to

400 °C. These have been attributed to the breaking of C-C linkages between the lignin

structural units (Ciobanu et al. 2004), as well as the fragmentation of inter-unit linkages

(Gordobil et al. 2015). The temperature of this stage for LS was markedly lower than that

of DLS, approximately in the range of 200 to 350 °C. This could be due to the release of

water, CO2 (Liu et al. 2008), CO, alkyls (Li et al. 2014), and possibly some SO2. As a

characteristic product of sulfonate compounds, the sulfonic acid groups pyrolyze at

temperatures lower than 200 °C (Van Loon et al. 1993). The presence of SO2 was

indeterminable at this step. The third degradation step of LS occurred above 350 °C, with

a substantial release of large molecular volatiles, aromatics, alkenes, and alkyls (Li et al.

2014). Additionally, the maximum weight loss rate in DLS was found to occur at 340 °C,

while that of LS occurred at a lower temperature of 238 °C. Moreover, the levels of

residue content at the end of the test for LS and DLS were found to be 62.72 ± 1.59% and

42.13 ± 2.73%, respectively. The higher char residue content of the LS has been

attributed to higher sodium salts and chemically bonded sulfur (Leger et al. 2010).

Fig. 3. (a) TG and (b) DTG curves of LS and DLS

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4817

Fig. 4. DSC curves of LS and DLS

The DSC curves of LS and DLS are shown in Fig. 4. Due to the complex structure

of lignin, the value of Tg was often difficult to measure. The Tg of lignins ranges from 90

to 180 °C in the literature (Li and McDonald 2014). In this study, it was remarkable that a

sharp, strong exothermic melt peak was observed at 150 °C for DLS instead of the more

typical glass transition at around 180 °C for LS. This could be associated with the lower

molecular weight of DLS after desulfonation.

Properties of PLA/Lignin Composites Thermal properties of PLA/lignin composites

Regarding the neat PLA and PLA composites with 5% and 10% LS and DLS, the

(a) TG and (b) DTG curves are shown in Fig. 5, respectively. In addition, the onset

degradation temperature (T5%), the maximum weight loss temperature (Tmax), and the

residue content are listed in Table 3.

Fig. 5. (a) TG and (b) DTG curves of neat PLA and the composites

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4818

Table 3. Onset Degradation Temperature (T5%), Maximum Weight Loss Temperature (Tmax), and Residue Content of Neat PLA and Composites

Sample Code T5% (°C) Tmax (°C) Residue (%)

PLA 312 365 0.9

PLA/LS5 239 319 4.1

PLA/LS10 234 314 5.6

PLA/DLS5 316 356 2.9

PLA/DLS10 322 360 3.9

The thermal degradation of PLA involved a single degradation stage and was

consistent with literature reports indicating that it occurs at 365 °C (Tran et al. 2014). The

shapes of the overall TG and DTG curves of PLA/DLS were similar to those of the neat

PLA. Composites including DLS displayed a slight increase in T5% but a similar Tmax

compared with PLA. However, the inclusion of LS resulted in a dramatic decrease of the

initial degradation and maximum decomposition temperatures compared with neat PLA

and PLA/DLS composites. The higher thermal stability of PLA/DLS than that of PLA/LS

was mostly due to the better thermal stability of DLS. Furthermore, the residue content

increased as DLS and LS were added into the composites.

In addition, the thermal properties of the neat PLA and PLA/lignin composites

were studied by DSC analysis (Fig. 6). The Tg increased in the PLA/DLS composites but

remained almost unchanged in the PLA/LS composites. The increase in the Tg with the

incorporation of DLS is an indication of the increased miscibility of this lignin with the

PLA, whereas the LS appears to be immiscible since the Tg did not shift. Also, the

endothermic peaks of samples at approximately 150 °C, associated with the melting point

of PLA, were not considerably impacted by incorporating LS or DLS in PLA.

Fig. 6. DSC curves of neat PLA and PLA/lignin composites

Crystallization of PLA/lignin composites

X-ray diffraction patterns of the PLA and PLA/lignin composites are presented in

Fig. 7. The XRD patterns of PLA reveal the typical diffraction peaks at 2θ = 14.76°,

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4819

16.7°, 19.1°, 22.4°, and 31.5°, which suggest that the layer distances of the PLA were

5.99, 5.31, 4.64, 3.96, and 2.84 nm, respectively.

The shape of the XRD pattern of the PLA/lignin composites was similar to that of

neat PLA, indicating that there was no apparent change in crystallinity as a result of the

introduction of the lignins as a nucleating agent. The same phenomenon has been

observed with lignin in PHB (Weihua et al. 2004). The d200 and d203 peaks of PLA were

noticeable in the PLA composites containing 10% LS and 5% DLS, which indicates that

LS and DLS could accelerate the crystallization of PLA with an appropriately low

content of lignin.

However, as the content of DLS became higher than 10%, all the peaks

disappeared. Other researchers have detected a similar phenomenon where the polymer

matrix was also reduced with increasing content of not only lignin, but also organoclay

(Kubo and Kadla 2003; Gordobil et al. 2014). This has been found to act as a retardant if

a strong interaction exists between the polymer matrix and the organoclay (Di et al. 2003;

Lee et al. 2008). In a related research (Liao et al. 2007), it was found that a negligible

amount of nucleating agent was present to help form the polymer crystal, whereas a

higher nucleating agent content hindered the ordered arrangement of the molecular chain

and resulted in low crystallinity.

Fig. 7. X-ray diffraction patterns of the PLA and PLA/lignin composites

Moreover, in the cases when the lignin content was below 10%, crystallization

was more prominent with the increase of LS, whereas the peaks became weaker with the

increasing addition of DLS into the PLA matrix. Meanwhile, the size of the LS was much

larger than that of the DLS, as illustrated in Fig. 8 (a) and (b). Such results appear to

contradict, at least in part, previous observations that a filler using smaller particle sizes

generally results in greater crystallinity (Tisserat et al. 2013). However, the diameter,

shape, surface, and properties of nucleating agents affect the crystallinity of PLA (Liao et

al. 2007). Therefore, the co-effects of size, content, and type of lignin particles acting as a

nucleating agent should be taken into consideration.

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4820

Fig. 8. SEM images of (a) LS and (b) DLS

Compatibility between PLA and lignin

The morphology of the lignin particles introduced into the PLA matrix is revealed

in Fig. 9 to illustrate the compatibility between PLA and lignin. As evident in Fig. 8 (b),

the size of the DLS particles (0.1 to 1 μm) became much smaller than that of the LS

particles (10 to 30 μm) shown in Fig. 8 (a). Both LS and DLS have a large size

distribution, due to the formation of inner molecular bonds with lignin molecules

(Teixeira et al. 2012; Chen et al. 2014b). For the PLA/lignin composites, the

homogeneous break surface of the neat PLA is shown in Fig. 9 (a). On the other hand,

cavities of lignin particles in the modified PLA matrix were detected and are shown in

Fig. 9 (b) and (c). The smaller size of lignin after desulfonation indicates the likelihood of

greater compatibility with lignin within the PLA (Gordobil et al. 2014).

Fig. 9. SEM images of the break surface of PLA/lignin materials: (a) neat PLA, (b) PLA/LS5 and (c) PLA/DLS5

Contact angle of PLA/lignin composites

The contact angle was investigated to determine the hydrophobicity of PLA/lignin

composites. The values of the contact angle of neat PLA and PLA/lignin composites are

shown in Fig. 10. The contact angle of PLA/LS was lower than that of neat PLA and

PLA/DLS. The hydrophilicity of the PLA/lignin composites was improved when a

predetermined amount of LS was introduced into the PLA matrix, which is consistent

with similar previously reported findings (Cazacu et al. 2016). However, the contact

angle of PLA/DLS was higher than that of neat PLA and PLA/LS, and it increased with

the growing content of DLS loadings. A better hydrophobicity was exhibited by the

PLA/DLS composites, which was attributed to the lower contents of strong hydrophilic

group (-SO3- ) in DLS.

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Fig. 10. Contact angle of neat PLA and PLA/lignin composites

Mechanical properties of PLA/lignin composites

Tensile tests were performed to study the effect of desulfonation on the

mechanical properties of PLA/lignin composites. The tensile strength at break, elongation

at break, and Young’s modulus of the PLA composites with different percentages of LS

or DLS are shown in Fig. 11.

The results reveal that PLA had better tensile strength compared with its

composites, and this finding is in agreement with previously reported results for

PLA/fiber composites (Razak et al. 2014). However, desulfonation mildly affected the

mechanical properties of PLA/LS composites in all conditions by producing only a small

decrease in maximum strength.

The slightly decreased mechanical strength in polymer/LS and PLA/lignin

composites have also been reported by other researchers (Chung et al. 2013; Duval et al.

2013; Gordobil et al. 2015). In our case, the slight reduction in maximum strength

resulted from irregular condensation and a random dispersion of tiny DLS spheres in the

PLA matrix (Fig. 9).

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4822

Fig. 11. Mechanical properties of neat PLA and PLA/lignin composites: effect of lignin content on (a) maximum tensile strength, (b) elongation at break, (c) Young’s modulus of PLA, PLA/LS, and PLA/DLS composites

Optical properties of PLA/lignin composites

The absorption of UV light is a desirable property for plastic materials, helping to

prevent damage and extend the shelf life of light-sensitive products (Ozdemir and Floros

2004; Agrawal et al. 2014; Heinrich et al. 2015; Lizundia et al. 2016). PLA/lignin

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4823

composites and pure PLA were used to investigate UV light absorption by UV-Vis

measurement.

The optical absorption spectra within the UV region (190 to 400 nm) for the

specimens are shown in Fig. 12. It is important to mention that absorbance of UV light

occurred at approximately 220 to 250 nm in all PLA/lignin specimens, and the UV

absorbance increased as lignin was introduced into the PLA matrix. Both PLA/LS and

PLA/DLS could potentially be applied to help protect light-sensitive products from UV

light damage.

Fig. 12. UV−Vis absorbance spectra of PLA/lignin composites

CONCLUSIONS

1. In this study, the basic structure of lignin remained after desulfonation. A marked

decrease in the Mw (For LS, Mw = 2329 g·mol-1; for DLS, Mw = 635 g·mol-1) and

sulfur content (DLS = 2.32 ± 0.11%, LS = 5.78 ± 0.58%) of LS was caused by

desulfonation. Moreover, the maximum weight loss rate upon heating of DLS was

much lower, which showed an improved thermal stability after desulfonation.

Additionally, the high char residue contents of LS and DLS were found to be 62.72 ±

1.59% and 42.13 ± 2.73%, respectively.

2. In addition, the PLA/lignin composites were successfully fabricated by the casting

method. The enhanced hydrophobicity of PLA/DLS composites after desulfonation

was measured through contact angle. The results revealed that PLA/DLS had a better

thermal stability (Tmax = 360 °C) than PLA/LS (Tmax = 314 °C). Meanwhile, the Tg

increased in PLA/DLS but was unchanged in PLA/LS composites. Also, the

endothermic peaks of PLA/lignin composites, associated with the melting point of

PLA, were not considerably impacted by incorporating LS or DLS in the PLA matrix.

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Ye et al. (2017). “Desulfonation of lignosulfonate,” BioResources 12(3), 4810-4829. 4824

Moreover, both LS and DLS acted as nucleating agents at low content (5% for

PLA/DLS and 10% for PLA/LS) in PLA. A greater compatibility between PLA and

DLS was detected as a result of the smaller size of lignin (0.1 to 1 μm) obtained after

desulfonation. However, the effect of desulfonation of LS was quite small with

respect to UV absorption and the mechanical properties of PLA/lignin specimens. A

strong UV absorption at 220 to 250 nm and almost unchanged mechanical properties

were detected in both PLA/LS and PLA/DLS composites.

ACKNOWLEDGMENTS

The authors are grateful for the financial support from the Co-built Project Beijing

Municipal Education Commission (2016) “R & D on Technology in Scientific Utilization

of Non-Wood Plant Material”.

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Article submitted: February 23, 2017; Peer review completed: April 6, 2017; Revised

version received and accepted: May 5, 2017; Published: May 16, 2017.

DOI: 10.15376/biores.12.3.4810-4829


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